Coiled tubing has been a useful apparatus in oil field drilling and related operations. A typical use is to measure or sample fluids downhole by placing one end of the coiled tubing into a borehole, lowering the end by unreeling the coiled tubing at the surface, obtaining the measurement or sample, and reeling the tubing end back up to the surface. Other applications have been to drill out scale and to provide a conduit during (non-drilling) open hole operations or cased hole workovers.
The potential to significantly reduce drilling costs by using coiled tubing instead of conventional drilling using drill pipe sections has been long recognized. Some of the potential cost saving factors include the running speed of coiled tubing units (which is normally much greater than conventional drilling rigs) and the reduced pipe handling time, pipe joint makeup time, and leakage risks using coiled tubing. Avoiding some drilling stops (e.g., to makeup a joint) by using coiled tubing can also reduce formation damage caused by interrupted mud circulation.
In spite of the significant potential cost savings by drilling with coiled tubing, only limited applications of coiled tubing to drilling and related processes are known. One application is to re-enter a vertical hole to deepen it over a relatively short distance (i.e., coiled tube drilling only the last and smallest portion). Another application is to re-enter a vertical hole to drill relatively short horizontal laterals. Completion applications of coiled tubing have been similarly limited.
The limited applications of coiled tubing are thought to be the result of problems normally experienced when using coiled tubing. One set of problems is related to the difficulties in trying to rotate coiled tubing. A conventional rotary drilling rig rotates the entire drill string from the surface (which rotates a rotary drill bit downhole), but because a portion of the coiled tubing remains on a drum (at the surface), a downhole motor is typically added near the bottom end of the coiled tubing to rotate the rotary drill head. The downhole motor adds complexity and cost.
Another set of problems is associated with the relatively thin walls of the coiled tubing. The thin walls are required to allow the tubing to be coiled on a reasonable diameter reel or drum at the surface (e.g., to be within maximum highway transport limitations). The thin walls limit differential pressures, rotational loads, hanging weight, and drilling forces that can be applied to the tubing. Thus, if a difficult-to-drill formation is encountered during the drilling of a well, drilling using coiled tubing may have to be abandoned with associated high costs. In part to compensate for these thin wall limitations, drilling with coiled tubing may be accomplished at higher rotational speeds. However, these higher speeds and the thin wall limitations typically require smaller diameter rotary drilling tools.
Another set of problems is associated with the residual bend in the coiled tubing. As the coiled tubing is uncoiled and straightened, a residual bend typically remains, i.e., most, but not all of the bend from coiling is removed. This residual bend can result in deviations from vertical (or the intended path) during drilling of the borehole. The amount of deviations may also change as different formations (and drilling loads) are encountered. The residual bend may also cause added contact and forces on the walls of the hole, resulting in increased frictional drag and an uncentered position of the tubing within the hole. The uneven loads can also cause early failure of the rotary drill bit.
Another set of problems is associated with the maximum tubing diameter. Although larger diameter tubing is available that could be coiled onto a large diameter drum, the larger drum size may not be highway transportable. The smaller (typically no more than 23/8 inch or 6.03 cm) tubing diameters have limited coiled tubing drilling applications to "slim holes." These slim holes may be later reamed out by conventional drilling, if required, but the two step drilling process is costly. The smaller tubing diameters similarly limit completion applications. In addition to the limited production flow (because of limited cross-sectional area) capability of the smaller tubing in completed wells, the tubing size may also limit the downhole use of fishing tools, logging tools, production pumping equipment, etc.
Many of these problems using coiled tubing are particularly acute for thermal injection well applications, such as steam injection wells. Unplanned drilling deviations for a single injection well can result in early breakthrough and loss of many other production wells. Injection steam flow and pressure requirements may be beyond the pressure and diameter limitations of coiled tubing. The residual bend and uncentered tubing location within the hole during completion operations can lead to excessive thermal losses during steam injection.